1. Field of the Invention
The technical scope of the invention is that of micro-machined or micro-engraved devices and more particularly time control devices for the movement of a counterweight incorporated into such micro-machined device.
2. Description of the Related Art
Over the past few years, miniaturized mechanical devices have been proposed for manufacture that incorporate micro-machined or micro-engraved electro-mechanical elements, either in an element deposited on a substrate, or directly on the substrate itself. This technology, known by the acronym of MEMS (Micro Electro Mechanical) today enables micro-mechanisms to be produced implementing a technique similar to that employed in the production of electronic integrated circuits.
Numerous types of devices are made using this technique, for example sensors, accelerometers, inertial units.
It is sometimes necessary in such a micro-mechanical device to ensure the time control or braking of the movement of a mechanical organ, for example, a counterweight subjected to the action of a motor, a spring or else subject to inertial stresses.
For several years it has been known, in particular, to produce safety and arming devices for the pyrotechnic train of a projectile made using such MEMS technology.
Certain designs of such safety devices incorporate a shutter to break the pyrotechnic train, whose plane is located perpendicularly to the direction of action of this train. Such a device is disclosed, for example, in patent EP1601926. The disadvantage with such a device lies in that the thickness of the MEMS shutter is not enough to stop a pyrotechnic effect. This leads to an electrical detonator with an exploding layer (known as a “slapper”) being associated with such a shutter, as described by U.S. Pat. No. 6,173,650. In this case the screen does not stop the pyrotechnic effect but prevents the projection of the plate intended to ensure the ignition of a detonation relay.
Other devices are also known in which the shutter does not break the pyrotechnic train but ensures the interruption of an optical control signal (for example, for the ignition of an explosive by laser beam). Such devices are known in particular by patents EP1559986, EP1559987.
Safety and arming devices are also known in which the shutter itself carries a pyrotechnic composition which it may introduce into the pyrotechnic train. Such devices are described, for example, by U.S. Pat. Nos. 6,622,629, 7,552,681, 7,490,552.
Lastly, devices are known in which the direction of action of the pyrotechnic train is substantially parallel to the plane of the shutter. These devices are disclosed by patents EP1780496 and EP2077431. The latter devices are particularly well adapted to safety devices for medium-caliber projectiles since they may be associated with percussion detonators.
For all known MEMS safety and arming devices the problem arises of the time control of a movement to displace a slide, a counterweight, or even the shutter itself.
It has been proposed, in particular by U.S. Pat. Nos. 6,568,329 and 6,064,013, to ensure this delay by providing slight play between the mobile counterweight and its housing and by arranging reliefs or indentations on the lateral faces of the counterweight and its housing. Given the play, when a stress appears (for example, inertial, axial or centrifugal acceleration), the indentations cooperate with one another to produce a “zigzag” movement of the counterweight, and thus a delayed or timed displacement. This is particularly useful when the arming of the device is sought to be delayed for the first 10 to 15 meters of the projectile's trajectory upon exiting the weapon, generally imposed by the safety rules applicable to projectiles.
This known solution suffers several drawbacks, however.
The zigzag movement of the counterweight is a mechanical movement that is difficult to control. This results in a degree of unreliability and a delay whose value cannot by guaranteed.
The parameters to be modified to adjust this delay are limited. An increase in the number of indentations in the counterweight will thus require the radial play to be increased thereby increasing the risk of the counterweight becoming blocked. A spring also often has to be added to control the movement of the counterweight, thereby increasing the device's complexity.